Saturday, January 17, 2009

Just what IS a species, anyway?

The title of this post might suggest to you that I'm about to launch into a philosophical discussion of species concepts. That's an interesting topic but not exactly what I have in mind. Even Darwin didn't know what a species was, and he identified dozens and dozens of barnacle species (but he missed the pink iguana [PNAS] [photo]). In Origin, he just settled for saying that naturalists knew what they meant when they used the term. Historically, there's kind of two ways that people have thought about species. On the one hand, the experiments of Francesco Redi really focused attention on the idea that species are reproductively defined. On the other, Linnaeus' work emphasized the idea of immutable units of creation. Obviously, the modern biological species concept follows the reproductive idea, and Linnaeus' idea of species fixity has been all but abandoned (even by me).

Baramins throw a monkey wrench into questions of species. If, as I believe, baramins are the fixed created units that Linnaeus was looking for, then what exactly are species? To put it another way, why have discrete species at all? Why not just have one extremely (and I do mean extremely) variable species per baramin? I wish I knew. I'm sure the answer to this question is not difficult and will appear quite obvious when someone discovers it. In the meantime, though, we can ponder a somewhat simpler question: How do species originate?

From an evolutionary perspective, species originate when two populations of one species become reproductively isolated. The kind of adaptive or morphological characteristics that we casually use to differentiate species can originate coincident with reproductive isolation or some time later. To a creationist who believes in rapid speciation after the Flood, the question of morphological adaptation must be caused by designed changes. Those changes can't be just random. So where does reproductive isolation come from? Is it part of the adaptation process, intended to keep the discrete species separate, even within the same baramin? Or is it merely something incidental that just happens by chance?

Given the widespread occurrence of interspecific hybridization (especially in angiosperms), I tend to suspect that reproductive isolation is just incidental. If keeping the species of a baramin discrete and separate was important, then it stands to reason that reproductive isolation would be strong. Instead, it seems that species are capable of interbreeding, but they don't. Or if they do, sometimes the hybrid offspring are sterile and sometimes they're fertile. That just seems more like a random change than something that was intended.

Two papers in the latest Science may help in unraveling this question. You can read about them in Willis's perspective article or in the original research articles (Mihola et al. and Phadnis and Orr). I'm not going to discuss the research findings in detail. You can read any of those three articles for that information. What intrigues me about the articles is that they trace hybrid sterility to single genes (one in mouse, one in Drosophila). The genes appear to be unrelated, and they work by different mechanisms.

I really like that. It fits well with the idea that reproductive isolation is just something that sort of happened. Not necessarily contrary to the Creator's plan, but not something that was specially endowed either. Adaptation, methinks, is the real key to understanding the post-Flood diversification of species within baramins. It will be interesting to follow these discoveries to see what kind of molecular evolution has brought about hybrid sterility.

Willis, JH. 2009. Origin of species in overdrive. Science 323:350-351.

Mihola, O, Trachtulec, Z, Vlcek, C, Schimenti, JC, and Forejt, J. 2009. A mouse speciation gene encodes a meiotic histone H3 methyltransferase. Science 323:373-375.

Phadnis, N and Orr, HA. 2009. A single gene causes both male sterility and segregation distortion in Drosophila hybrids. Science 323:376-379.